Even the Quran, which Sufis respect as the direct speech of God, lacks the capacity to shed light upon God's essence. As one Sufi master has argued, why spend time reading a love letter (by which he means the Quran) in the presence of the Beloved who wrote it?
No god but God (Updated Edition): The Origins, Evolution, and Future of Islam (ed. Random House, 2011) - ISBN: 9780679643777